If you got 403 error when installing WordPress on addon domain try to disable security plugins on your main domain to have access to htaccess files and have all the permissions.
Follow the wisdom of Tech Tzu:
I’m trying to install WordPress on my add-on domain but if you have your main domain, on your main domain, if you have add on like for example All in one WordPress security. This add-on this plugin will prevent the access to the htaccess file and you will not be able to install WordPress on a add-on domain . And as you can see, up here would you like All in one WordPress security and firewall to re-insert the security rules and those security rules that All in one WordPress security and firewall insert into my htaccess file, my main htaccess file was the main problem. And that’s it guys say it’s a pretty simple solution, I thought it was in phpconfig file and…